Effects of forest litter on dissolved 137Cs concentrations in a highly contaminated mountain river in Fukushima

Study region: The upper, forested reaches of the Ota River, 24.4 km northwest of the Fukushima Daiichi nuclear power plant, was studied. Study focus: There is concern about the effects of dissolved radiocesium (137Cs) released from forest litter on long-term contamination of aquatic ecosystems caused by the Fukushima nuclear accident. Therefore, hydrology and water quality measurements were performed under base flow and storm flow conditions in a high-dose-rate forest watershed in Fukushima Prefecture, to investigate the involvement of litter in 137Cs release. New hydrological insights for the region: Under base flow conditions, the dissolved 137Cs activity concentration significantly positively correlated with the dissolved organic carbon and potassium ion concentrations (the main components leached from litter) over the entire study period. Differences in the annual variations in the 137Cs activity concentrations and potassium ion concentrations indicated that litter indirectly increased 137Cs concentrations in river water by releasing potassium ions that caused 137Cs to desorb from river sediment and riparian soil. In each large runoff event, the dissolved 137Cs activity concentration was higher at the time of maximum runoff than at the start of the event. The relationships between the dissolved 137Cs, dissolved organic carbon, and potassium ion concentrations strongly suggested that 137Cs leached from litter in runoff was directly responsible for the increase in the 137Cs activity concentration in river water.